The ability to remotely sense parameters of interest in people and objects has long been desired. Presently, various monitoring technologies are known and used to sense conditions or to provide identification in a wide range of contexts. One such technology, known as "tagging," is commonly employed, for example, in shoplifting security systems, security-badge access systems and automatic sorting of clothes by commercial laundry services. Known tagging systems frequently use some form of radio-frequency identification (RF-ID). In such systems, RF-ID tags and a tag reader (or base station) are separated by a small distance to facilitate near-field electromagnetic coupling therebetween. Far-field radio tag devices are also known and used for tagging objects at larger distances (far-field meaning that the sensing distance is long as compared to the wavelength and size of the antenna involved).
The near-field coupling between the RF-ID tag and the tag reader is used to supply power to the RF-ID tag (so that the RF-ID tag does not require a local power source) and to communicate information to the tag reader via changes in the value of the tag's impedance; in particular, the impedance directly determines the reflected power signal received by the reader. The RF-ID tag incorporates an active switch, packaged as a small electronic chip, for encoding the information in the RF-ID tag and communicating this information via an impedance switching pattern. As a result, the RF-ID tag is not necessarily required to generate any transmitted signal.
Even though RF-ID tags have only a small and simple electronic chip and are relatively inexpensive, the solid-state circuitry is still relatively complex and vulnerable to failure. Another limitation of conventional monitoring techniques is the type of stimuli that can be sensed and the degree of sensing that can be performed. For instance, known LC-resonator sensing systems rely on macroscopic mechanical changes in the material structure, which indirectly leads to a change in the capacitance. For example, a foam-filled capacitor may be used to sense forces. As the capacitor is squeezed, its capacitance and, hence, the resonance frequency changes in response to the force. Such systems are not only relatively thick, but are also limited to sensing stimuli that affect the stress-strain curve of the dielectric. Also, the dynamic range of such systems is limited by the modulus of the dielectric; because of the difficulty in making extremely thin materials that can be squeezed, an effective lower limit is placed on the thickness of the capacitor. Accordingly, a need exists for an enhanced sensing system capable of monitoring a variety of stimuli (such as temperature, humidity and/or light) in addition to force.